1. Field of the Invention
This invention relates generally to the field of current sense circuits, and more particularly to high-side current sense circuits.
2. Description of the Related Art
There are numerous applications in which it is necessary to measure a particular current in a circuit. For example, it is often desirable to know the current consumed by a circuit. This could be determined by sensing the current on the “high-side” of the circuit—i.e., where a non-zero supply voltage is connected to the circuit, or on the circuit's “low-side”—i.e., where the current returns to its source.
When measuring a high-side current, the signal of interest has a non-zero DC voltage component which can prove problematic for some current measurement circuits. An example of this is illustrated with the circuit shown in FIG. 1, which measures the current using a sense resistor having a resistance Rsense connected in series with the signal whose current (I) is to be measured. Current I causes voltages V2 and V1 to be developed on either side of Rsense, which are provided to the inputs of a differential amplifier A1 powered by supply voltages VCC and VEE. When configured as shown, the output (Vo) of the amplifier is given by:Vo=(Rb/Ra)(V2−V1)=I*Rsense(R2/R1).The voltage I*Rsense is differentially measured and amplified by Rb/Ra. The common mode voltage at the input of A1 is given by:V+=V−=V2[Rb/(Ra+Rb)].
Ideally, the voltages V+ and V− at the input of the amplifier are within the range of its power rails VCC and VEE. However, if the voltages V1 and/or V2 are outside the power rails, the ratio Rb/Ra may be forced to be less than 1, which in turn forces the output voltage (Vo) and the measured current value to be reduced. This general approach is used, for example, in the ADM1041 Secondary-Side Controller with Current Share and Housekeeping IC from Analog Devices, Inc., and is also discussed in U.S. Pat. No. 6,617,838 to Miranda et al.